The present invention relates to a coin-shaped battery suitable for power sources and the like in thin electronic devices, and to a method for producing such a coin-shaped battery.
Coin-shaped organic electrolyte batteries comprise elements for electromotive force comprising a positive electrode featuring the use of manganese dioxide or the like, a negative electrode featuring the use of lithium or an alloy thereof, and an electrolyte featuring the use of an organic electrolyte, which are housed in a flat battery case also serving as the positive electrode terminal. A characteristic feature of such coin-shaped batteries is the high energy density resulting from the use of lithium in the negative electrode active material. Such batteries not only allow devices to be made smaller and more light-weight, but are also highly reliable, and are thus used as the main source of power in various types of electronic devices and as memory back up power sources. Particularly in recent years, the ability to make thinner electronic device casings has been accompanied by increasing demand to make thinner coin-shaped batteries, which are housed in such devices. The structure of a conventional coin-shaped battery is described below.
FIG. 12 depicts the cross sectional structure of a coin-shaped organic electrolyte battery in which manganese dioxide is used for the positive electrode and metallic lithium is used for the negative electrode. In the figure, reference numeral 21 is a metal case serving also as the positive electrode terminal; 24 is the positive electrode mix comprising a compression molded mixture of manganese dioxide, graphite, and a binder; 26 is a separator comprising a polypropylene nonwoven fabric; 25 is the negative electrode metallic lithium; 22 is a metal sealing plate generally in the shape of a dish, which also serves as the negative electrode terminal; and 23 is an insulating gasket. There have been studies on making such coin-shaped batteries thinner by changing the shape of the various aforementioned structural components, that is, by making the various structural components thinner.
The aforementioned insulating gasket is generally produced by injection molding, primarily using resins such as polypropylene. To make thinner gaskets by injection molding, it is necessary to reduce the gap through which the resin flows in the mold. However, due to the low fluidity of such resins in molten states, the resin is not sufficiently packed into the more detailed sections of the mold, which can result in resin deficiencies, leading to so-called short shot. In gaskets affected by such short shot, the parts with insufficiently packed resin are thinner than the other parts. As a consequence, coin-shaped batteries using such gaskets are not sealed as well in parts where the case is crimped, resulting in drawbacks such as leakage. The ability to produce thinner gaskets featuring the use of a resin such as polypropylene is thus limited, making it extremely difficult to achieve a thinness of about 0.2 mm or less.
Other problems with thin gaskets formed by injection molding are that the resin shrinks after molding, and that thinner gaskets suffer from inadequate strength, resulting in deformation during transport and battery assembly. Still other problems that occur when gaskets are placed on top of each other are that the gaskets are difficult to pick up during the manufacturing process, resulting in lower productivity.
Thin gaskets thus produced by injection molding suffer from the aforementioned drawbacks. It is thus difficult to make thinner coin-shaped batteries using thinner gaskets, and only coin-shaped batteries that are about 1.2 mm thick can be mass produced.
Furthermore, since the various components forming such thinner coin-shaped batteries are all smaller and thinner, they are more susceptible to breakage and deformation while handled, and must therefore be manually assembled with great care. The gasket 23 which is used to seal the portions where the battery case is sealed by crimping the sealing plate 22 and the battery case 21 or the separator 26 is a thin resin component which tends to deform and is troublesome to handle, thus complicating assembly and acting as a major impediment in the productivity and cost reduction of thin coin-shaped batteries.
In the process disclosed in U.S. Pat. No. 5,603,157 as a means for resolving such drawbacks, a UV curable resin is applied in the form of a thin ring using a screen printing technique to the surface of a thin metal sheet forming the case of the coin-shaped battery, the resin is then irradiated with UV rays and cured, and a disk that has a diameter which is concentric with the aforementioned UV curable resin applied in the form of a ring but which is somewhat larger than the ring-shaped resin is then punched from the electrode case metal sheet coated with the aforementioned UV curable resin and is drawn to produce a coin-shaped battery case with the gasket attached. In this case, extremely high productivity may be anticipated, yet it may also be assumed that it would be difficult to produce a uniform UV curable resin ring of the prescribed thickness consistently in the radial direction, and it may furthermore be assumed that UV curable resins capable of withstanding lithium battery non-aqueous electrolytes are neither currently unavailable for commercial purposes nor likely to be developed in the future.
An object of the present invention is to improve the structure of a gasket suitable for use in coin-shaped batteries so as to provide a coin-shaped battery with better battery properties and productivity, particularly a coin-shaped battery with a thickness of no more than 1.0 mm, as well as a method for producing such a coin-shaped battery.
In the coin-shaped battery manufacturing process of the present invention, the gasket is supplied to the battery assembly step in the form of an integrated component with the gasket assembled on the battery case, thereby eliminating smaller and thinner parts that are susceptible to deformation from the assembly step, while simultaneously reducing the number of parts that are supplied to the assembly step, reducing the number of assembly steps and the number of steps for controlling parts, as well as using parts with stable shapes that are easy to handle, in an effort to promote the total automation of the assembly step.
To achieve the aforementioned object, the present invention provides a coin-shaped battery comprising elements for electromotive force sealed therein by crimping the periphery of a sealing plate and the internal periphery of a metal case, with a gasket comprising an insulated resin being interposed therebetween, wherein the gasket comprises a resin film formed into a bottomed cylindrical shape with a round hole in the bottom, the aforementioned gasket being crimped while integrated by adhesion to the inside surface of the case. The gasket preferably comprises a resin selected from any of polyphenylene sulfide (PPS), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), and polypropylene (PP).
In the present invention, the resin film preferably ranges in thickness from 20 xcexcm to 150 xcexcm, the adhesive used to integrate the gasket to the inside surface of the case is preferably any of styrene butadiene rubber, butyl rubber, polyolefin, styrene propylene, fluorine, or epoxy adhesives, and the battery preferably ranges in thickness from 0.2 mm to 1.0 mm.
The present invention has greater effect when used as an organic electrolyte battery in which metallic lithium is used as the negative electrode.
Because the gasket is formed by reshaping the resin film into the bottomed cylindrical shape in the present invention described above, it can be made thinner more easily than gaskets obtained by conventional injection molding. In addition, a film having a uniform thickness is used to avoid resin deficiencies that cause short shot during injection molding. Batteries can thus be made thinner to more easily obtain coin-shaped batteries with a thickness of no more than 1.0 mm.
To achieve the aforementioned object, the present invention also provides a coin-shaped battery, wherein elements for electromotive force and a sealing plate are disposed on a gasket-integrated case, the gasket integrated case being obtained by punching a circular piece from a metal sheet on which a gasket resin film having a round hole is integrally laminated with an adhesive, in such a way as to have a diameter concentric with, but larger than, the aforementioned round hole, and by drawing the circular piece into a bottomed cylindrical shape. The periphery of the aforementioned sealing plate is sandwiched and sealed by the periphery of the aforementioned case by crimping, with a gasket made of the aforementioned resin film interposed therebetween.
In a preferred method for producing such a coin-shaped battery, round holes matching the inside diameter of a gasket are punched from a resin film intended to serve as the gasket, with an adhesive previously applied on the side to be brought into contact with a metal sheet; the resin film is then placed on the surface of the metal sheet which has been processed into a case so that the two are laminated by adhesion by means of the adhesive; disks that have a shape concentric with, but larger than, the round holes are then punched from the resin-laminated metal sheet; the disks are then drawn to form gasket-integrated cases; and the gasket-integrated cases are used to produce coin-shaped batteries.
Because this invention allows a case with an integrally laminated gasket to be processed, products (gaskets) which tend to be deformed when handled are avoided, while the number of parts supplied to the assembly process are also reduced, allowing more consistent quality to be achieved and costs to be reduced.
Furthermore, as described above, round holes matching the gasket inside diameter are processed before the resin film is laminated to the metal sheet, so as to eliminate the need for additional processing after the battery case has been completed by drawing the punched disk, as well as to avoid irregular resistance in the battery by doing away with the need to apply an adhesive to the surface of the battery case in contact with the positive electrode mix after the disk has been punched from the resin film, since an adhesive has been provided on the resin film.
An uncured heat-sensitive adhesive may be applied in advance to one surface (the side brought into contact with the metal sheet forming the battery case) of the resin film that is to serve as the gasket. The two parts may be allowed to adhere to each other by being pressed as the adhesive is heated just before the film is laminated to the metal sheet, in order to allow the step for applying the adhesive, where handling the adhesive tends to cause staining, to be managed separately from the battery case processing device, and also to allow the performance of the heat-sensitive adhesive and the like to be controlled separately in advance.
In the present invention, the resin film preferably has a thin layer mainly composed of aluminum on one side, and is preferably caused to adhere to a metal sheet by means of an adhesive applied to the thin layer and/or metal sheet. In a particularly preferred embodiment, the adhesive is mainly composed of a polyolefin resin and has maleic acid-modified functional groups.
This can ensure greater adhesion between the resin film and metal sheet, and thus between the gasket and case.
To achieve the object described above, the present invention also provides a coin-shaped battery comprising elements for electromotive force sealed therein by crimping the periphery of a sealing plate and the internal periphery of a metal case, with a gasket comprising an insulated resin being interposed therebetween, wherein the gasket comprises a resin film formed into a bottomed cylindrical shape with a round hole in the bottom, the aforementioned gasket being integrated by adhesion to the inside surface of the case; and elements for electromotive force and sealing plate are then disposed on the gasket-integrated case, and the periphery of the sealing plate is then sandwiched and sealed by the periphery of the case by crimping, with the gasket interposed therebetween. A preferred embodiment of a method for producing such a coin-shaped battery is a method for producing a coin-shaped battery by providing a sealing plate at the opening of a metal case housing elements for electromotive force, and crimping the plate, with a gasket comprising an insulating resin fitted to the internal periphery of the case, wherein the method comprises the steps of: forming a bottomed cylindrical component by embossing a resin film; forming a round hole in the center of the bottom of the bottomed cylindrical component; punching the bottomed cylindrical component from the resin film to obtain a gasket; and fitting the gasket to a case on which an adhesive has been applied, so as to integrate the gasket and the case.
A means for integrating the aforementioned gasket by adhesion to the inside surface of the case is to use a styrene butadiene rubber, butyl rubber, polyolefin, fluorine, or epoxy adhesive, or to use an asphalt adhesive such as pitch which softens when heated and has adhesion for metals and resins.
The aforementioned embossing process is preferably carried out at a temperature at or beyond the glass transition point of the resin film. The step for punching the bottomed cylindrical component from the resin film to obtain a gasket can be carried out before the gasket is fitted into the case, but can also be carried out at the same time that, or after, the gasket is fitted into the case.
According to the aforementioned invention, a gasket can be formed without using injection molding by forming a portion of a flat resin film into a bottomed cylindrical shape by an embossing process, then punching a round hole in the center of the bottom of the bottomed cylindrical component, and cutting the border line between the flat sheet portion of the resin film and the bottomed cylindrical portion, such as in the aforementioned manufacturing method, for example. This allows a gasket of constant thickness to be produced without resin deficiencies that result when resin is inadequately packed during injection molding, as in conventional examples.
In the aforementioned invention, the gasket preferably has a thin layer mainly composed of aluminum on one side, and is caused to adhere to the case by means of an adhesive applied to the thin layer and/or metal case. In a particularly preferred embodiment, the adhesive is mainly composed of a polyolefin resin and has maleic acid-modified functional groups.
Such a structure overcomes the following problems which can occur when the gasket and case are made to directly adhere to each other. That is, in methods where the case and gasket are allowed to adhere to each other and are integrated by means of an adhesive, inadequate adhesion between the gasket and case can result in displacement of the gasket from the position where it originally adheres as well as in its separation from the case, potentially leading to accidents such as internal shorts and leakage. Such accidents are particularly a problem in batteries featuring the use of organic electrolytes. That is because materials with excellent organic solvent resistance such as polypropylene, polyethylene naphthalate, polyethylene terephthalate, and polyphenylene sulfide are used for the gasket material, making adhesion between the gasket and stainless steel case difficult. In the aforementioned structure of the present invention, such problems are overcome because adhesion is brought about between the metal case and the thin layer mainly composed of aluminum, that is, adhesion is brought about between metals, thereby ensuring that the gasket and case are joined in a firm and stable manner.